Power transformer having shields for shaping the electric field in the major insulation spaces

ABSTRACT

A power transformer having at least one shield positioned in the insulation structure of the transformer and located between the high-voltage and the low-voltage windings. The shield includes a conducting member which shapes the electric field in the insulation structure in a manner which permits more efficient utilization of the insulating members in the insulation structure. The shield may be connected to an intermediate tap on the high-voltage winding in order to acquire a fixed potential thereon.

United States Patent [191 Robin 1 Oct. 29, 1974' POWER TRANSFORMERHAVING SHIELDS FOR SHAPING THE ELECTRIC FIELD IN THE MAJOR INSULATIONSPACES [75] Inventor: Harral T. Robin, Muncie, Ind.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Dec. 19, 1973 21 Appl. No.2 426,390

52 U.S.'C1. .Q. 336/84 [51] Int. Cl. H01t 15/04 [58] Field of Search336/69, 70, 84

[56] References Cited UNITED STATES PATENTS Biermanns 336/70 FOREIGNPATENTS OR APPLICATIONS 1,513,911 4/1969 Germany 336/84 415,414 0/1934Great Britain 336/70 1,474,535 2/1967 France 336/70 1,213,911 4/1966Germany 336/84 Primary Examiner-Thomas .1 Kozma Attorney, Agent, orFirmJ. R. Hanway [57] ABSTRACT A power transformer having at least oneshield positioned in the insulation structure of the transformer andlocated between the high-voltage and the lowvoltage windings. The shieldincludes a conducting member which shapes the electric field in theinsulation structure in a manner which permits more efficientutilization of the insulating members in the insulation structure. Theshield may be connected to an intermediate tap on the high-voltagewinding in order to acquire a fixed potential thereon.

1 Claim, 8 Drawing Figures .PATENIEBUBTZQ 191 4 3.845.436 SHE 10$ 3 PRIFIG.

FIG. 2

FIG. 4

POSlTlON ON HIGH-VOLTAGE- WINDING STRUTURE PATENTEDnm 29 I974 SHEEIZOF 3llllllllllllllllllllllllllllll III PATENTEDnm 29 I974 samura mvmw-POSITION ON VERTICAL BARRIER SURFACE FIG.8

POSITION ONHORIZONTAL FIG 6 BARRIER SURFACE O O O O O EIEJVIIOA H OBIHJJM (SW8 HONl/AN) S53E18 dI-THO BACKGROUND OF THE INVENTION 1. Field ofthe Invention:

This invention relates, in general, to electrical inductive apparatusand, more specifically, to power transformers having shields for shapingthe electric field within the insulation structure of the transformer.

2. Description of the Prior Art:

Large power transformers, particularly of the shellform type, require aconsiderable amount of insulating material positioned between theprimary and secondary windings of the transformer and between thewinding structures and the magnetic core. The use of solid insulatingmaterial, such as pressboard, is desirable due to its excellentdielectric properties and the ease with which it may be assembled intothe transformer. However, since cooling oil must circulate through theinsulation structure to cool various components of the transformer, oilspaces or channels between the pressboard members must be provided.Therefore, some portions of the insulation structure have oil spaces orvoids which do not contain any solid insulating material. Other portionsof the'insulation structure contain regions which are completelyoccupied by solid insulating material since the stresses in theseregions may become high enough to cause insulation failure if anappreciable oil space is present.

According to the prior art, the pressboard insulating members arearranged in a manner consistent with the electric field which may existin the insulation structure. As a result thereof, some regions of theinsulation structure contain more than one thickness of pressboardmaterial, some of the oil spaces are larger than others, and some of theregions of the insulation structure contain substantially a solid massof pressboard insulating members. Because of the relatively rigid natureof the pressboard material and the general shape of the windingstructure in large shellform power transformers, the insulationstructure usually consists of numerous pressboard sheets mounted eitherhorizontally or vertically within the winding structure. Since this typeof construction technique must normally be used, the arrangement of theinsulation structure to provide the necessary dielectric strength isvery time consuming and a surplus of solid insulating material in someregions of the insulating structure usually occurs.

Typical electrical fields in large shell-form power transformers haveequipotential lines which pass through the insulation structure and forman oblique angle with the surfaces of the pressboard insulating members.Due to the angle of the equipotential lines of the electric field,voltage differences between points on the surface of the pressboardmembers and between the opposite sides of the pressboard members aredeveloped. Thus. the pressboard is susceptible to both creepage andpuncture failure if the electrical stresses are too large. In order tomake the most efficient use of the pressboard insulating members, it isdesirable to have the equipotential lines aligned parallel with thesurface of the pressboard member. Thus, it is desirable, and it is anobject of this invention, to provide a means for shaping the electricfield in the insulation structure of a power transformer, with theresultant shape of the field permitting a more convenient and efficientuse of the solid insulating material in the insulation structure of thetransformer.

SUMMARY OF THE INVENTION There is disclosed herein a new and usefularrangement for shaping the electric field within the insulationstructure of a power transformer. At least one shield, consisting of anelectrical conducting member, is located within the insulation structureof the transformer and positioned between a high-voltage winding and alow-voltage winding of the transformer. The conducting member may beconnected to a potential within the high-voltage winding structure ofthe transformer. Due to the conducting nature of the shield, theequipotential lines in the portion of the insulating structure betweenthe high-voltage winding and the low-voltage winding acquire asubstantially straight shape which conforms to the orientation of thepressboard insulating members contained in this region of the insulationstructure. The shield extends sufficiently into the portion of theinsulation structure which is located between the high-voltage windingand the magnetic core of the transformer in order to shape theequipotential lines existing in this portion of the insulationstructure. These equipotential lines extend generally in a perpendiculardirection from the shield and are substantially parallel to thehorizontal insulating members in the in sulation structure. The overallresult of the shaping of the electric field is that the equipotentiallines assume a more rectangular shape than equipotential lines in ainsulation structure constructed according to the prior art. Thus, sincethe solid insulating members are substantially positioned in theinsulation structure in a rectangular fashion, more efficient usethereof is made when the shield of this invention is used.

BRIEF DESCRIPTION OF THE DRAWING Further advantages and uses of thisinvention will become more apparent when considered in view of thefollowing detailed description and drawing, in which:

FIG. 1 is a schematic representation of high-voltage and low-voltagewindings in a shell-form power transformer constructed according to theprior art;

FIG. 2 is a schematic representation of the highvoltage and low-voltagewindings in a shell-form power transformer constructed according to thisinvention;

FIG. 3 is a partial cut-away view of a shell-form power transformerconstructed according to this invention;

FIG. 4 is a graph illustrating the difference in the puncture stressesnear the high-voltage winding structure of a transformer constructedaccording to the prior art and of a transformer constructed according tothis invention;

FIG. 5 is a graph illustrating the difference between the creep stressesat Hz on a horizontal barrier surface of a transformer constructedaccording to the prior art and of a transformer constructed according tothis invention;

FIG. 6 is a graph illustrating the difference between the creep stressesat 180 Hz on a vertical barrier surface of a transformer constructedaccording to the prior art and of a transformer constructed according tothis invention;

FIG. 7 is a graph illustrating the difference between the creep stressesdue to an impulse voltage on avertical barrier surface of a transformerconstructed according to the prior art and of a transformer constructedaccording to this invention; and

FIG. 8 is a graph illustrating the difference between the creep stressesdue to an impulse voltage on a horizontal barrier surface of atransformer constructed according to the prior art and of a transformerconstructed according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout the followingdescription, similar reference characters refer to similar elements ormembers in all the figures of the drawing.

Referring to the drawing, and to FIG. 1 in particular, there is shownschematically a shell-form power transformer 10 constructed according tothe prior art. The primary or high-voltage winding 12 and the secondaryor low-voltage windings l4 and 16 contain coil sections, such as thecoil section 18, which are positioned in inductive relationship witheach other. The coil sections are disposed around a portion of themagnetic core 20 which is not illustrated in FIG. 1. The shellform typemagnetic core 20 also extends around the ouside of the coil sections. Astatic plate 22 is positioned adjacent to the coil section 18 andimproves the impulse voltage distribution across this end of thehighvoltage winding 12.

When a voltage is applied to the high-voltage winding 12, an electricfield develops between the high-voltage winding 12 and the low-voltagewindings l4 and I6, and between the high-voltage winding 12 and themagnetic core 20. The dashed lines 24, 26, 28, 30 and 32 indicate theposition of equipotential lines which exist between the high-voltagewinding 12 and the lowvoltage winding 14 and the magnetic core 20 when alow-frequency voltage, such as a 60 or I80 H2 voltage, is applied to thehigh-voltage winding 12. Although shown as lines in the two dimensionaldiagram of FIG. 1 for simplicity, the equipotential points locatedwithin an actual transformer 10 would form a three dimensional surfacewhich extends around the high-voltage winding structure 12. The electricfield pattern existing in the transformer 10 is unique from other typesof transformers due to the difference in size of the highvoltage coilsections.

With the voltage on the low-voltage winding 14 and the core 20 being atsubstantially ground potential, a low-frequency voltage applied to thehigh-voltage winding 12 provides an equipotential line 24 whichrepresents 10% of the voltage on the static plate 22. The equipotentialline 26 represents percent of the voltage, the line 28 represents 50percent of the voltage, the line 30 represents 75 percent of thevoltage, and the line 32 represents 90 percent of the voltage. From theequipotential lines illustrated in FIG. I, it can be seen that by usingsubstantially rigid and straight insulating members in the insulationstructure, it would be impractical to provide an insulation structurewhich makes the most efficient use of the solid insulating material.

FIG. 2 is a schematic representation of the transformer shown in FIG. 1with a field shaping shield disposed therein as taught by thisinvention. In this specific embodiment, the transformer 34 includes theelectrically conducting shields 36 and 38 as shown in FIG. 2. Theshields 36 and 38 are positioned between the high-voltage winding 12 andthe low-voltage winding 14 and are orientated substantiallyperpendicular to the axis of the winding structure. The shields 36 and38 in this embodiment are constructed similar to the static plate 22,but with larger outside dimensions. Details of the construction ofstatic plates to which the shields 36 and 38 may be similar aredescribed in US. Pat. Nos. 3,376,53I and 3,643,196, both of which areassigned to the same assignee as is this invention. In general, theshields exhibit a hollow rectangular shape. It is within thecontemplation of this invention that other arrangements for providingthe conducting element in the shields 36 and 38 may be used.

The shield 38 is electrically connected by the lead 40 to the coilsection 42 at position 44. The equipotential line 46 extends from oneend of the shield 38, through the coil section 42, to the other end ofthe shield 38. Since a constant potential exists on the conductingelement of the shield 38, the electric field in the region of the shield38 is substantially straight and is orientated vertically according tothe direction illustrated in FIG. 2.

In addition to straightening the electric field in the region adjacentto the shield 38, the electric field in the insulation structure aboveand below the high-voltage winding 12 is straighter than the field inthe same region of the transformer shown in FIG. 1. Control of the shapeof the field in these regions is determined by the outside dimensions ofthe shield 38, its spacing from the high-voltage winding 12, and thepotential to which it is connected. In the specific embodimentillustrated in FIG. 2, the shield 38 is substantially the same size asthe coil section 42 to which it is connected.

The shield 36 is connected by the lead 48 to the coil section 50 atposition 52. The shield 36 is larger than the shield 38 and shapes theequipotential line 54 between the shield 36 and the high-voltage coilsection 50. As can be seen from an overall view of FIG. 2, theequipotential lines exhibit a more rectangular shape with the placementof the shields 36 and 38 in the portion of the insulation structurewhich is located between the high-voltage winding 12 and the low-voltagewinding 14. Thus. efficient and convenient use of the solid insulatingmembers may be made in the transformer insulation structure. Moreshields than the two indicated, or only one shield, may be used withoutdeparting from the scope of the invention.

By properly selecting the position at which the shields are located, theleads connecting them to the proper coil section in the high-voltagewinding 12 may be tapped to an outside turn of the coil section fortapping convenience. Thus, the potential on the shield would besubstantially equal to the potential at the outside of the coil sectionto which it is connected. In addition, it is possible to shape theelectric field in the insulation structure without connecting theshields, such as the shields 36 and 38, to a potential within thehighvoltage winding 12. With such a construction, a potential developson the shield which is related to the position of the shield between thehigh-voltage and the lowvoltage windings and the insulating materialtherebetween.

FIG. 3 is a partial cut-away view of a shellform power transformerconstructed according to this invention with one electric field shapingshield. The magnetic core 20 is separate from the high-voltage windingstructure 12 by the portion of the insulation structure which includesthe horizontal pressboard members 54. The low-voltage windings wouldnormally be positioned on both sides of the high-voltage windingstructure 12 and would be separated therefrom by the vertical pressboardmembers 56. The coil section 18 is electrically connected to the staticplate 22 by a lead which is not shown in FIG. 3. A shield 58 ispositioned between the high-voltage winding 12 and the low-voltagewinding which is not shown, and is oriented substantially in parallelwith the vertical pressboard members 56.

The structure shown in FIG. 3 makes it possible to make the mostefficient use of the pressboard insulating members contained in theinsulation structure. Since the puncture and creep stresses on thepressboard members are reduced in many portions of the insulatingstructure, less solid insulating material may be used and still providesufficient electrical insulation. Thus, by conforming the electricalfield to convenient and standard insulating structures, a moreeconomical insulating structure may be constructed than by trying toconform the insulation structure to the electric field normallyexhibited by such transformers. By using less insulating material, theoverall size of the transformer may be reduced.

FIG. 4 is a graph illustrating the decrease in the puncture stressadjacent to the coil sections of the highvoltage winding structure 12 bythe addition of a stress shaping shield. Data collected for this graphwas obtained from a transformer constructed with a shield positioned inthe mid-point of the high-low space, or half way between thehigh-voltage winding 12 and the lowvoltage winding 14 as shown in FIG.2. The shield was also connected to a potential within the windingstructure 12 which was equal to the voltage at the position of theshield during low-frequency tests without any field shaping shieldpresent. Curve 62 represents the puncture stress in a transformer havinga sheild for shaping the electric field. Curve 60 represents thepuncture stress in a similar transformer without such a shield. Atdifferent positions within the high-voltage winding structure. thepuncture stress exhibited by the transformer having the shield wasalways lower than the puncture stress in a similar transformer withoutsuch a shield. The positions A through L designated in FIG. 4 refer tothe coil sections of the high-voltage winding as they progress in adirection away from the static plate.

FIG. 5 is a graph representing the creep stress on a horizontal barriersurface or pressboard member in transformers with the application of a180 Hz voltage. Curve 64 represents the creep stress on a horizontalbarrier surface in a transformer with the shield, and curve 66represents the creep stress at the same position in a transformerwithout the shield. As shown in FIG. 5, the creep stress at lowfrequency voltages on the horizontal barrier surfaces is considerablyless with the transformer containing the electric field shaping shield.

FIG. 6 illustrates the creep stress under similar conditions on verticalbarrier surfaces, or pressboard members. Curve 68 represents the creepstress as measured in the transformer containing the electric fieldshaping shield and curve 70 represents thecreep stress at the sameposition without the shield.

FIG. 7 indicates the creep stress on a vertical barrier surface, orpressboard member, when an impulse voltage is applied to the testtransformers. The creep stress is expressed in percent of the appliedimpulse voltage. Curve 72 represents the creep stress for thetransformer with the shield placed therein and curve 74 represents thecreep stress without the shield. It can be seen from FIG. 7 that, withan impulse voltage applied to the high-voltage winding, the creep stresson the vertical barrier surface is substantially less in the transformerhaving the electric field shaping shield.

FIG. 8 represents the difference in creep stress on a horizontal barriersurface as a percent of the applied impulse voltage. Curve 76 representsthe creep stress in the transformer with the shield and curve 78represents the creep stress in the transformer without the shield.

FIGS. 4 through 8 are typical of curves measured at other positionswithin the winding structure of the transformer and illustrate aconsiderable reduction in the electrical stress to which the solidinsulating members of the insulation structure are subjected. It isemphasized that the shield placed between the highvoltage and thelow-voltage winding structures of the transformer do not necessarilygrade the potential be tween the winding structures, but actually shapethe electric field in the portions of the insulation structure which arelocated between the high-voltage winding structure and the magneticcore. It is also emphasized that the shields do not act primarily ascapacitor plates in grading electrical stresses between and on sides ofthe plates, but act as an electrical means for shaping the electricfield surrounding the high-voltage windings, particularly the fieldbetween the high-voltage winding and the magnetic core. This isaccomplished by increasing the height of the equipotential lines as theyenter the insulation region between the high-voltage winding and themagnetic core.

Since numerous changes may be made in the above described apparatus, andsince different embodiments of the invention may be made withoutdeparting'from the spirit thereof, it is intended that all of the mattercontained in the foregoing description, or shown in the accompanyingdrawing, shall be interpreted as illustrative rather than limiting.

I claim as my invention:

1. A shell-form power transformer comprising:

a shell-form magnetic core;

a primary winding structure and a secondary winding structureinductively coupled to said magnetic core, said primary windingstructure including a plurality of substantially rectangular coilsections disposed at different axial positions, with the coil sectionclosest to said secondary winding structure having a smaller number ofturns than the other coil sections;

a non-magnetic static plate positioned adjacent to the coil section ofthe primary winding structure which has the smaller number of turns;

a first non-magnetic, electrical conducting shield positionedsubstantially parallel to said static plate and between said staticplate and said secondary winding structure, said shield having a largeroutside dimension than said static plate; and

a second non-magnetic, electrical conducting shield positionedsubstantially parallel to said first shield and between said firstelectrical conducting shield and said secondary winding structure, saidsecond shield having a larger outside dimension than said first shield,with said first and second shields being electrically connected topotentials within the primary winding structure.

1. A shell-form power transformer comprising: a shell-form magnetic core; a primary winding structure and a secondary winding structure inductively coupled to said magnetic core, said primary winding structure including a plurality of substantially rectangular coil sections disposed at different axial positions, with the coil section closest to said secondary winding structure having a smaller number of turns than the other coil sections; a non-magnetic static plate positioned adjacent to the coil section of the primary winding structure which has the smaller number of turns; a first non-magnetic, electrical conducting shield positioned substantially parallel to said static plate and between said static plate and said secondary winding structure, said shield having a larger outside dimension than said static plate; and a second non-magnetic, electrical conducting shield positioned substantially parallel to said first shield and between said first electrical conducting shield and said secondary winding structure, said second shield having a larger outside dimension than said first shield, with said first and second shields being electrically connected to potentials within the primary winding structure. 